The present invention relates to noise control for gas turbine engines, and more particularly to fan exit guide vanes having acoustic impedance control features for attenuating or dampening noise in a gas turbine engine.
Gas turbine engines often include a bypass duct, especially engines used for commercial aerospace applications. A fan assembly can draw air into the engine, and a portion of that air is diverted through the bypass duct. Fan exit guide vanes (FEGVs) extend into the bypass duct downstream of the fan assembly. These FEGVs provide an aerodynamic function in straightening or otherwise interacting with airflow from the fan assembly, and a structural function in delivering mechanical support in a generally radial direction across the bypass duct.
However, noise produced by gas turbine engines is a concern. Noise generated by fan-wake/vane interaction is a significant contributor to the effective perceived noise level (EPNL) of gas turbine engines. Such noise problems can occur when wakes of the upstream fan assembly impinge on the FEGVs, thereby providing a mechanism for converting non-acoustic vortical disturbances (i.e., the fan wake) into propagating pressure disturbances (i.e., sound).
A number of prior art solutions have been proposed to address these concerns. For instance, active noise control using vane-mounted actuators, and use of vane sweep and lean to introduce radial phase cancellation are known, but such systems are often undesirably complex. Also, acoustic bypass duct liners are known for dampening noise along the fan bypass duct, but such liners can be undesirably large. In addition, engine systems can be designed to mitigate fan-wake/vane interaction by doing the following: (a) increasing axial spacing between the fan assembly and the FEGVs to increase decay of a wake velocity deficit, (b) selecting a sufficiently high ratio of FEGVs to fan assembly blades such that, for the associated blade-passing frequency, spinning modes generated by the fan-wake/vane interacting are “cut-off” and cannot transmit acoustic power all the way through the length of the bypass duct, and (c) utilizing a relatively long bypass duct. These solutions undesirably constrain fan assembly, fan case and FEGV structural design, and can dictate a longer bypass duct that would otherwise be desired.